1. Field of the Invention
The present invention relates to a printing press, and more particularly to improvement in ink feed control.
2. Description of the Background Art
Recently, printing press having a main body including a plate making device for forming an image on a printing plate on the basis of digital image data are being widely used, which are known as a CTP (computer-to-plate) machine. A First conventional printing press is called a digital printing press, which is suited to short jobs for printing operations of multiple types and small number of printed sheets because printed sheets are directly obtained from the image data. In the digital printing press, the plate making process is automated so that it can be handled easily by an inexperienced operator, but further automation is demanded in ink supply control in the printing process.
In the ink feed control in the conventional printing press, generally, a color measurement console connected to the printing press is used, but in this case, the operator has to pick up proper print samples to measure the color of the printed sheet. By the color measurement console, usually, a color chart provided in the print sheet is measured.
To solve this problem, a second conventional printing press having means for detecting an image of a print sheet has been developed. In this second prior art, the image of the printed sheet is detected by an image detector provided on an impression cylinder of the printing press to obtain image data. This image data is compared with reference image data obtained preliminarily by reading a reference image, to control the ink feed rate. According to the second prior art, since the image of the print is detected in the printing press, the operator is not required unlike the case of using a color measurement console. Another advantage is that the color chart is not necessary because the image on the print sheet is used.
In the second prior art, however, the reference printed sheet must be prepared. Such printed sheet is a proof sheet or a favorable sheet previously obtained during printing process, which is known as xe2x80x9can OK-sheetxe2x80x9d. Lately, however, proof sheets are often obtained by a simple proof press using an ink jet printer or the like, and a reference printed sheet to be used as a reference in the printing press is not always prepared beforehand. In the method making an OK-sheet during printing, the ink feed must be controlled manually until an OK-sheet is obtained. Thus, the method is applicable only in mass printing, and it is not economical in time and number of printed sheets in printing operations for obtaining a small number of printed sheets. It is hence in appropriate in the digital printing press suited to printing operations of multiple types and small number of printed sheets.
Further, in the second prior art, means for detecting the image of a printed sheet is provided near an impression cylinder. In this case, it is difficult to detect the image of the printed sheet over the entire area. That is, when detecting the image of the rear end of the printing sheet, the leading end is already caught by another cylinder (for example, discharge cylinder), and the rear end of the printing sheet is not fixed to the impression cylinder. In such a case, along with the move of the printing sheet, the rear end side fluctuates and the image thereof may not be detected properly.
Further, as in the second prior art, in a satellite type printing press in which plurality of blanket cylinders contacting with the impression cylinder, enough space for disposing the image detecting means may not be provided near the impression cylinder.
The invention is directed to a printing press for printing an image on a printing sheet on the basis of first image data.
According to the invention, the printing press comprises plate making means for forming an image on a printing plate on the basis of the first image data, ink feed means capable of changing ink feed rates of ink supplied onto a plurality of regions of the printing plate, reference color data operating means for converting the first image data into a predetermined color coordinate system for each one of the plurality of regions, and storing color values obtained as reference color data, image-detecting means for detecting an image on the printing sheet transferred from the printing plate to thereby obtain second image data, print color data operating means for converting the second image data into the color coordinate system for each one of the plurality of regions to obtain print color data, differential data operating means for comparing the reference color data with the print color data for each one of the plurality of regions to obtain differential data, correction data operating means for obtaining correction data for the ink feed rates on the basis of the differential data, and control means for controlling the ink feed rates to be fed onto the plurality of regions by the ink feed means on the basis of the correction data.
In this printing press, the ink feed can be automated without preparing the reference print sheet.
In other aspect of the invention, the printing press comprise plate making means for adding a color chart image data expressing a predetermined color chart to the first image data to generate combined image data, and forming an image corresponding to the combined image data on a printing plate, sink feed means capable of changing ink feed rates of ink supplied onto a plurality of regions of the printing plate, reference color data storage means for storing a numerical value of each color included in the color chart in a predetermined color coordinate system as reference color data, image-detecting means for detecting an image on the printing sheet transferred from the printing plate to thereby obtain second image data, print color data operating means for converting the second image data into the color coordinate system for each one of the plurality of regions to obtain print color data, differential data operating means for comparing the reference color data with the print color data for each one of the plurality of regions to obtain differential data, correction data operating means for obtaining correction data of the ink feed rate on the basis of the differential data, and control means for controlling the ink feed rates to be fed into the plurality of regions by the ink feed means on the basis of the correction data.
Preferably, the printing press further comprises color chart storage means for storing the color chart image data. The plate making means adds the color chart image data being read out from the color chart storage means to the first image data, and generates the combined image data.
In a preferred embodiment of the invention, the printing sheet has a leading end and a rear end. The printing press further comprises discharge means for delivering the printing sheet by gripping the leading end, and stabilizing means for stabilizing a conveying state of the printing sheet being discharged. The image-detecting means detects the image on the printing sheet stabilized by the stabilizing means in the conveying state.
In a preferred embodiment of the present invention, the printing press further comprises a discharge storage for stacking up and accumulating a plurality of printed sheets sequentially on a platform. The image-detecting means has two-dimensional image-detecting means capable of detecting a whole image region of a top printing sheet of a plurality of printing sheets stacked up on the discharge storage.
The invention is also directed to a method of controlling the ink feed rate in the printing press.
According to the invention, the method comprises the steps of obtaining reference digital color data by converting first digital data expressing colors into a predetermined color coordinate system, providing a printing mechanism with color inks corresponding to the first digital data, transferring the color inks onto a printing sheet from the printing mechanism, obtaining second digital data by detecting colors on the printing sheet, and controlling color ink feed rates in the printing mechanism in response to a difference of the first data and the second digital data by a feedback control.
Accordingly, an object of the invention is to provide a printing press capable of automatically controlling an appropriate ink feed rate without preparing any reference print.
It is other object of the invention to detect an image favorably even in the rear end of a printed sheet by disposing imaging means in other area than the vicinity of the impression cylinder.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.